The present invention relates to a process for the manufacture of expanded polymeric polystyrene products. The process of the present invention is directed at a time-efficient method of making expanded polymeric products while simultaneously utilizing a low level of blowing agent for making a low density foam product. This is achieved by using: (1) a very low level of blowing agent, in combination with both (2) multiple premolding expansion steps, as well as (3) a polymer which is highly expandable, in combination with an aging time within a specified time period. The final result is a process which not only utilizes less blowing agent, but also can be made time-efficient, and which can result not only in cost savings, but more importantly, reduced release of environmentally-damaging volatile organic compounds into the atmosphere.
Methods for the production of molded polymeric products have required making an intimate mixture of a polymer and a blowing agent. In commercial operations, this intimate mixture was generally made into solid (relatively "high-density") beads of relatively small size (e.g. beads having a diameter of from about 0.2 to 4 millimeters). The beads were then expanded (via heating the mixture) in order to make the foam. The expansion was usually carried out by heating the beads to a temperature at which a polymer gel formed, resulting in the vaporization of the blowing agent. (The blowing agent must have a boiling temperature below the softening temperature of the polymer.) The vaporization of the blowing agent caused an expansion of the beads to form individual particles of foam. The expansion was generally carried out by using a first expansion step, whereafter the now-expanded particles (referred to as prepuff) were then placed into a mold and again heated, whereby the prepuff further expanded and, because of the confined volume, fused to form a unitary object. Under optimal conditions, the bonds formed between the individual prepuff particles were stronger than the individual particles themselves. That is, upon stressing the finished, molded object enough to cause it to break, the break would occur within the individual "cells", rather than at the junctions of the cells.
Fundamental to the foaming operation is the requirement that the polymer contain a blowing agent. In prior commercial production, steam has been used to heat the beads, the steam causing the blowing agent to vaporize, which vaporization results in the formation of gaseous bubbles within the polymer. These bubbles expand as the temperature increases, resulting in the formation of a foam Since the vast majority of the volume within the foam is occupied by vapor, rather than polymer, the product formed has a density much lower than that of the unexpanded polymer/blowing agent mixture.
A large fraction of the foamed polystyrene currently being produced has a density of from 0.9 lb./sq.ft. to 1.1 lb./sq.ft. This density of material is generally used for insulation and/or protective packaging end uses. In order to achieve this density, it has been the practice of the commercial manufacturers of such "low-density" foamed polystyrenes to incorporate from about 6 to about 9 weight percent of pentane (the most common blowing agent used today) into the polystyrene polymer.
Beads are formed via a suspension polymerization during or after which pentane is introduced into the beads. These beads are then heated in a single premolding expansion step in which the beads are expanded in volume by a factor of about 40 (i.e. to a density of about 1.0 lb./cu.ft.). The now preexpanded beads are allowed to cool and equilibrate by permitting air to diffuse thereinto, and are then put into a mold where they are again heated, resulting in the further expansion of the preexpanded prepuff, so that the prepuff particles are bonded together.
In the process of the present invention, it has been surprisingly found that the length of time required to adequately "cure" the expanded bead (i.e. before further expansion of the bead) can be significantly reduced in comparison with prior art processes.
U.S. Pat. No. 2,884,386 describes a process for making cellular bodies of organic thermoplastic materials The process described therein involves making an intimate mixture of a blowing agent with a thermoplastic resin and thereafter expanding the mixture to form a cellular thermoplastic body. The specification refers to the use of cycles of expanding operations that, if used repeatedly, cause further expansion of the prepuff particles made in accordance therewith. However, the '386 patent nowhere provides any general statement as to how many premolding expansion steps should be utilized.
The '386 patent nowhere provides any general description of the amount of blowing agent to be used in the process. However, the '386 patent does state that the preferred primary blowing agent is dichlorodifluoromethane, and the Examples in the '386 patent all utilize only halogenated methanes as the blowing agent. Example 1 of the '386 patent utilizes 2 weight percent dichlorodifluoromethane with eight expansion steps to achieve an undisclosed final product density. Example 2 utilizes 8.11 weight percent dichlorodifluoromethane with nine expansion steps to effect a 150.times. volumetric increase (the density of the final product was undisclosed). Example 4 utilized 20 volume percent of dichlorodifluoromethane in a three step expansion process, to effect a final product density of 0.938 lb./cu ft. In comparison with the process of the present invention, these examples (as well as the remaining examples of the '386 patent) utilize such a high level of blowing agent (and/or such a high number of expansion steps) that the process of the present invention is not only not suggested, the process of the present invention is also taught away from.
The '386 patent also fails to provide one with the unexpected result of the present invention: i.e. that if one were to use merely from 2 to 4.4 weight percent of a blowing agent, a product of relatively low density (i.e. from 0.8 to about 1.1 lb/cu.ft.) could be produced with only from 2 to 5 expansion steps. The gist of the '386 patent is the generalized notion that multiple expansion steps can be used to effectuate a volumetric increase greater than the theoretical volumetric increase possible from the expansion of the blowing agent alone. As the '386 patent states repeatedly, this increase is brought about allowing a more permeable secondary blowing agent, such as air, to diffuse into the foam whereupon after cooling an additional heating will produce further expansion due to the presence of this secondary blowing agent in further heating/expansion steps Although the process of the present invention certainly utilizes this mechanism of increasing the degree of expansion, the process of the present invention is directed towards a specific area wherein this mechanism is used in addition to other critical process steps, i.e. the use of a low (2-4.4 weight percent) level of blowing agent, in combination with the use of only 2 to 5 expansion steps, as well as the use of a specific polymer type (i.e. a polymer exhibiting three characteristics: (1) a polydispersity of from about 1 to less than 2.5; (2) a weight average molecular weight of from greater than 180,000 to about 300,000; (3) a Mz:Mn of from about 2 to about 4.5; and (4) is branched to from 0 to 5 weight percent). It should be noted that the '386 patent nowhere discloses a polymer having such characteristics.
Furthermore, the process of the present invention isolates a specific area of improvement over the subject matter disclosed in the '386 patent. The process of the present invention relates to the use of beads of thermoplastic polymer which contain only from about 2 weight percent to about 4.4 weight percent of a hydrocarbon blowing agent. It has surprisingly been found that even with such a small amount of the blowing agent, the bead can be expanded to a final density of from about 0.8 to about 1.1 lb./cu. ft., while using only 2 to 5 expansion steps (or 2 to 4 preexpansion steps before the molding step). The '386 patent nowhere achieves such final product densities while utilizing so little blowing agent.
Applicants have discovered that the use of a low amount of blowing agent provides many important advantages, among which are:
(1) a reduction in the amount of blowing agent required, resulting in cost savings; PA1 (2) reduced environmental pollution since less blowing agent (generally a volatile organic compound, i.e. a VOC) is released into the environment both during manufacture and during consumer use; PA1 (3) processing advantages such as: PA1 (1) insufflating (i.e. pre-expanding to produce a partly expanded product) polystyrene granules containing blowing agent, the insufflating being carried out with steam at about atmospheric pressure, whereby the granules are partially expanded; PA1 (2) conditioning the partially expanded granules at atmospheric pressure; PA1 (3) subjecting the partially expanded granules (in a confined space) to steam at about 150 g./sq. cm. pressure; and PA1 (4) restoring the expanded granules to atmospheric temperature and pressure. PA1 (1) subjecting (for a few minutes) polystyrene granules to steam at low pressure; PA1 (2) conditioning the granules for a few hours at about 20.degree. C. to 40.degree. C.; PA1 (3) reheating the expanded granules to about 100.degree. C. with hot air; PA1 (4) then treating the granules with steam for 30 to 40 seconds; followed by PA1 (5) conditioning the granules for 1 to 24 hours. PA1 (1) partially expanding the polymeric particles; followed by PA1 (2) crushing the particles; followed by PA1 (3) again partially expanding the particles. PA1 (a) pre-expanding raw polystyrene beads containing a blowing agent in a pre-expansion vessel; PA1 (b) storing the beads in one or more closed storage containers to allow the internal pressure within the expanded beads to return to substantially atmospheric pressure; PA1 (c) molding the expanded beads to a desired configuration in a closed mold with steam; and PA1 (d) removing the thus-formed article from the mold and placing such in an aging room, wherein at each stage the blowing agent released from the beads is recovered, separated from any residual steam by means of a condensing system, and introduced into the burner of a steam generator, thereby serving as fuel for the process. PA1 pentane (including isomers of pentane such as cyclopentane, neopentane, isopentane, as well as pentane petroleum distillate fractions), methylcyclopentane, propane, butane, isobutane, hexane, isomers of hexane, 2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane, 2,3-dimethylbutane, cyclohexane, methylcyclohexane, heptane, propylene, 1-butylene, 2-butylene, isobutylene, mixtures of one or more aliphatic hydrocarbons having a molecular weight of at least 42 and a boiling point not higher than 95.degree. C. at 760 millimeters absolute pressure, water, carbon dioxide, ammonium carbonate, and azo compounds that are decomposable to form a gas at a heat-plastifying temperature to which the polymer is brought. PA1 styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenated vinyltoluenes which form linear polymers, acrylonitrile, and methyl methacrylate. PA1 pentane (including isomers of pentane such as cyclopentane, neopentane, isopentane, as well as pentane petroleum distillate fractions), methylcyclopentane, propane, butane, isobutane, hexane, isomers of hexane, 2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane, 2,3-dimethylbutane, cyclohexane, methylcyclohexane, heptane, propylene, 1-butylene, 2-butylene, isobutylene, mixtures of one or more aliphatic hydrocarbons having a molecular weight of at least 42 and a boiling point not higher than 95.degree. C. at 760 millimeters absolute pressure, water, carbon dioxide, ammonium carbonate, and azo compounds that are decomposable to form a gas at a heat-plastifying temperature to which the resin is brought. PA1 styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenated vinyltoluenes which form linear polymers, acrylonitrile, and methyl methacrylate.
(a) a lower shrinkage in the molding step; PA2 (b) quicker cooling in the molding step, resulting in shorter processing times; PA2 (c) shorter aging time between the preexpansion steps and between the last preexpansion step and the molding step, resulting in shorter processing times; and PA2 (d) ability to be easily molded with acceptable fusion and dimensional stability on molding while using a low level of blowing agent.
U.S. Pat. No. 4,839,396 describes a process for making expandable alkenyl aromatic polymer particles. These particles have the ability to use a decreased amount of blowing agent while maintaining the potential to produce a bulk density equivalent to that achieved by particles comprising a greater amount of blowing agent. This is achieved through the use of from 0.005 to 0.5 weight percent of a "density modifier". The '396 patent describes the density modifier as a compound providing thermal stability for the alkenyl aromatic polymer at extrusion and expansion conditions and which is also a liquid plasticizer at expansion conditions. These density modifiers are stated to include octadecyl 3 ,5-di-tert-butyl-4-hydroxyhydrocinnamate, as well as ethylene bis(oxyethylene)bis(3-tert-butyl-4-hydroxy-5-methylhydrocinnamate).
The specification of the '396 patent states that the "volatile fluid foaming agents" (i.e. the blowing agents) usually are employed in amounts corresponding to from about 5 to about 15 percent of the weight of the total formulation. The only examples in the '396 patent utilize from 8.8 to 10.3 weight percent of the blowing agent These examples show that the amount of the blowing agent was reduced from a level of 10.0-10.3 weight percent down to about 8.8 weight percent (i.e. a reduction of about 12 to 15 percent in the amount of blowing agent used), while achieving the same final density as in the comparative run having the greater amount (i.e. 10.0-10.3 weight percent) of blowing agent present.
The process of the present invention also has as a goal the reduction in the amount of blowing agent used in the manufacture of an expanded polymer. However, relative to the '396 patent, the process of the present invention permits at least as much as twice the percentage reduction in the amount of blowing agent (e.g. most preferably a reduction of from about 6 weight percent to about 3.5 weight percent, which is approximately a 40 percent reduction). The process of the present invention produces this comparatively large reduction in blowing agent via an approach which is different from the approach taken in the '396 patent. This approach is the use of an initially low level of the blowing agent (i.e. a blowing agent level of from 2 to 4.4 weight percent) while simultaneously using multiple preexpansion steps. It should be noted that although the '396 patent suggests the use of multiple preexpansion steps before the molding step, the '396 patent fails to make any connection between the use of low amounts of blowing agent and the use of multiple preexpansion steps, and the '396 patent has a very broad, undirected disclosure of the amount of blowing agent which can be used. Furthermore, the '396 patent suggests the use of only relatively high amounts of blowing agent (i.e. 5 to 15% broadly, with examples limited to from 8.8% to 10.3% by weight).
U.S. Pat. No. 4,520,135 and 4,525,484 (a divisional of the application filed for the '135 patent) are both directed at a polystyrene particles containing a blowing agent, wherein the polystyrene has an improved expandability. More particularly, these polystyrene particles are comprised of a polymer which has a molecular weight of from about 130,000 to about 180,000. The '484 patent describes several methods for making this polymer, i.e. via the use of chain transfer agents, the use of oligomers, or polymerizing in the presence of the blowing agent. The '484 patent states that the resulting polystyrene particles can be expanded by conventional methods (e.g. steam expansion). The '484 patent states that the blowing agent can be present in an amount generally from 3 to 12 weight percent (preferably from 5 to 8 weight percent, and the only example in the '484 patent utilizes approximately 7 weight percent pentane) However, the '484 patent nowhere makes any statement regarding the use of multiple premolding expansion steps. In contrast, the process of the present invention utilizes a polymer which is different from the polymer described in the '484 patent in that it exhibits three characteristics which are different from the polymer described in the '484 patent. Foremost among these differences is the fact that the weight average molecular weight of the polymer used in the process of the present invention is higher than that of the polymer described in the '484 patent. Surprisingly, this higher molecular weight polymer has a degree of expandability at least as high as the polymer described in the ' 484 patent. In further contrast, the process of the present invention requires the use of from 2 to 4 premolding expansion steps, whereas the '484 patent makes no mention of multiple expansion steps either with or without molding.
U.S. Pat. No. 4,485,193 describes a process for making resilient foam particles and moldings with a "lightly crosslinked" polymer, which could be a styrene polymer. The process entails the use of a volatile fluid foaming agent that has low permeability through the polymer, and the process also uses multiple expansion steps for the production of foams of low density, i.e. suitable for molding. The process described and claimed in the '193 patent requires that the once-expanded particles are subjected to a superatmospheric pressure of at least 3 atmospheres in air, whereafter the now "pressurized" particles are further expanded by heating the particles above the glass transition temperature of the polymer after the particles are returned to a normal atmospheric pressure.
The '193 patent nowhere provides any generalized description of the amount of blowing agent which may be employed in the process described therein. However, of the 43 Samples discussed in the '193 patent: Samples 1-17 contained blowing agent in an amount of from about 20 to about 30 weight percent; Samples 18-21 were foams made "in accord" with the specification of the '193 patent but no data was provided re the amount of blowing agent employed in the making of the foam; Sample 22 was a foam made in accord with the conditions of Sample 2 (and Sample 2 utilized 28.9 weight percent blowing agent); Samples 23-38 contained blowing agent in an amount of from 6.12 to about 7.0 weight percent; Samples 39-41 do not contain any description of the amount of blowing agent, but merely state that blowing agent was permitted to diffuse into the already-formed beads; and Samples 42 and 43 appear to utilize blowing agent in an amount of at least 11 weight percent. In summary, the '193 patent teaches the use of blowing agent in an amount which is considerably higher than the amounts involved in the present invention.
U.S. Pat. No. 3,639,551 describes a cyclic method for producing low-density polystyrene foam beads, wherein the beads are expanded in a plurality of expansion steps. However, the gist of the '551 patent is that in between the expansion steps the now partially-expanded beads are reheated to restore the majority of the "lost volume" (i.e. the volume lost upon the cooling of the beads immediately after they were expanded). This reheating step precedes the next expansion step. By this method, the shrinkage of the beads will be prevented from substantially affecting the ultimate degree of expansion obtained.
The '551 patent nowhere provides any discussion of the quantity of blowing agent to be employed in expanding the beads. The '551 patent has a single example which states that:
1000 pounds of a commercial grade acrylonitrile styrene copolymer beads having low boiling hydrocarbon propellant (pentane) included therein and a diameter between about 1/64 to about 1/32 inch were stored in a hopper and fed into a "Buccaneer" preexpander (available from TRI Manufacturing & Sales Co., Lebanon, Ohio) having an expansion chamber substantially as described above. PA0 The starting material consisting of granules of polystyrene containing (sic) a pentane petroleum fraction as a blowing agent [Col. 2, lines 61-63]
The '551 patent nowhere states how much of the low boiling propellant (pentane) was present in the beads.
In stark contrast to the '551 patent, the process of the present invention utilizes a low level of blowing agent (from about 2 to about 4.4 weight percent) in combination with multipass expansion in order to achieve a product having a density of from about 0.8 to 1.1 pounds per cubic foot. The '551 patent nowhere mentions the use of such an unconventionally low amount of blowing agent. The process of the present invention further requires the use of a specific polymer, which polymer the '551 patent nowhere discloses.
U.S. Pat. No. 3,631,133 describes a process for expanding polystyrene beads in order to produce a bead having an exceptionally low density (i.e. 5 kg./cu. meter, or less, which equals approximately 0.3 lb./cu. ft., or less). The method described in the '133 patent is generally described as:
The gist of the '133 patent is that of using multiple expansion steps in combination with a conditioning step, in order to ultimately produce a low density product. The '133 patent nowhere refers to the quantity of blowing agent to be utilized in the process. Rather, all the specification (including Examples) has to say about the blowing agent is:
In stark contrast to the '133 patent, the process of the present invention utilizes a low level of blowing agent in combination with multipass expansion in order to achieve a product having a density of from 0.8 to 1.1 pounds per cubic foot. The '133 patent nowhere mentions the use of such an unconventionally low amount of blowing agent. Furthermore, the '133 patent nowhere refers to the characteristics of the polystyrene polymer used therein as providing anything other than a conventional level of expandability.
U.S. Pat. No. 3,598,769 describes a process for expanding polystyrene, this process involving:
The objective of the '769 patent is to provide a process for producing beads of polystyrene having an apparent specific mass less than about 7 kg./cu. meter (i.e. a density of about 0.44 lb/cu.ft., or less). The gist of the '769 patent is to provide a very specific process for using two of expansion steps and a conditioning step after each expansion step. Furthermore, the '769 patent is directed at carrying out this process on a continuous conveyor belt.
As with the '133 patent, the '769 patent nowhere describes the amount of blowing agent to be used in the process.
In stark contrast to the '769 patent, the process of the present invention utilizes a low level of blowing agent in combination with multipass expansion in order to achieve a product having a density of from 0.8 to 1.1 pounds per cubic foot. The '769 patent nowhere mentions the use of such an unconventionally low amount of blowing agent. Furthermore, the '769 patent nowhere mentions the use of a polymer having an extraordinary degree of expandability.
U.S. Pat. No. 3,126,432 describes a process for producing super-low density thermoplastic foam, namely polystyrene foam. The process described in the '432 patent involves expanding particles of polystyrene having a vaporizable liquid (butane or pentane) inflating agent therein, and thereafter aging the expanded particles first at atmospheric pressure and thereafter at superatmospheric air pressure (2 to 8 atmospheres) for several hours. This exposure to superatmospheric air pressure has the effect of causing a secondary blowing agent to migrate into the expanded particles. Thereafter, the pressure is released and within five hours the particles are heated in a closed mold. Thus the gist of the '432 patent is to "pump up" the expanded particles by exposing the particles to superatmospheric air, and thereafter carrying out a second expansion step by taking advantage of the relatively high internal pressure within the particles, once they are released from the pressure chamber. The '432 patent nowhere has any general discussion of the amount of blowing agent to be utilized in making polystyrene foams. Of the five examples given in the '432 patent, only Examples I, III, and V provide any information as to the amount of blowing agent used in the process. In each of these Examples, the blowing agent used is pentane, and the pentane is present in an amount of 6% by weight of the polystyrene globules. Thus it is clear that the '432 patent does not teach towards any process which utilizes a blowing agent in an amount less than 6% by weight.
In stark contrast to the '432 patent, the process of the present invention utilizes a low level of blowing agent in combination with multipass expansion in order to achieve a product having a density of from 0.8 to 1.1 pounds per cubic foot. The '432 patent nowhere mentions the use of such an unconventionally low amount of blowing agent. Furthermore, the '462 patent nowhere mentions a polymer exhibiting the properties of the present invention.
U.S. Pat. No. 3,056,753 describes the production of expandable polymeric particles having a foamed polymeric structure. The process described therein involves:
The gist of the '753 patent is to decrease the molding cycle time, whereby molded articles can be removed from the mold after permitting a cooling period of lower duration. Nowhere in the '753 patent is there any mention of the amount of blowing agent to be utilized in the process. In fact, even the seven examples within the '753 patent fail to provide any information as to the amount of blowing agent utilized. In stark contrast to the '753 patent, the process of the present invention utilizes a low level of blowing agent in combination with multipass expansion in order to achieve a product having a density of from 0.8 to 1.1 pounds per cubic foot. The '753 patent nowhere mentions the use of such an unconventionally low amount of blowing agent. Furthermore, the '753 patent nowhere mentions a polymer having the characteristics of the polymer of the present invention.
U.S. Pat. No. 4,721,588 describes a closed circuit process for the production of expanded polystyrene foam. This process comprises the steps of:
As can be seen from the above description of steps, the gist of the process described in the '588 patent is the recovery of the blowing agent and its re-use as a fuel for the heating step. This produces the dual effects of (1) reducing the amount of volatile organic compounds released into the atmosphere, as well as (2) obtaining a double use for the blowing agent which escapes from the polystyrene during the pre-expansion and molding steps.
The '588 patent also mentions that the blowing agent can be n-pentane, or mixtures of n- and iso-pentane (up to about 25% iso-pentane by weight). The '588 patent also states that the initial blowing agent content of the expandable polystyrene beads can be 4-8 weight percent. However, the '588 patent nowhere states that a low density foamed polystyrene product can be obtained if one utilizes less than 5 weight percent of the blowing agent. In fact, aside from the statement that a blowing agent content of 4-8 weight percent can be used, the '588 patent makes absolutely no mention of actual product densities or an further mention of the amount of blowing agent in any actual composition. Finally, the '588 patent makes no mention of the use of multiple pre-molding expansion steps. Rather, the '588 patent teaches one to simply perform one pre-molding expansion step and to thereafter follow this step with the molding step.
In contrast to the '588 patent, the process of the present invention utilizes a low level of blowing agent in combination with multipass expansion in order to achieve a product having a density of from 0.8 to 1.1 pounds per cubic foot. The '588 patent nowhere mentions the use of the combination of a level of blowing agent less than 5 weight percent with multipass expansion in order to achieve a product density of from 0.8 to 1.1 pounds per cubic foot. Furthermore, the '588 patent nowhere mentions a polymer having the characteristics of the polymer used in the process of the present invention.
In recent years the emissions of volatile organic compounds (i.e. VOC's) have come under increasing scrutiny by the EPA, state and local air quality boards as mandated by the Clean Air Act of 1977. Because hydrocarbon emissions have been shown to contribute to photochemical smog, the expanded polystyrene industry which uses pentane as a blowing agent has come under pressure to limit its use and/or emissions of pentane.
Since the early months of 1990, the inventors' process has enjoyed a high level of commercial success, with sales of at least 2 million dollars of a formulation which has been expanded to 0.8-1.1 lb./cu. ft. only with the inventor's process (which corresponds with sales of approximately 3 million pounds of the formulation), which formulation has a highly-expandable polymer present in an amount of about 96 weight percent, based on the total weight the formulation. Thus there has been a high level of commercial success of both the process as well as the formulation utilized in practicing the process.
For several years BASF Corporation has been involved in the manufacture and sale of a polystyrene product having approximately 6 weight percent pentane therein. This product had a polydispersity of 2.2, a weight average molecular weight of about 190,000, and an Mz:Mn of about 3.5. In stark contrast, the product of the present invention has a polydispersity of from 1 to less than 2, a weight average molecular weight of from about 200,000 to about 300,000, and an Mz:Mn of from about 2 to less than 3.
One polymer which has been commercialized for several years has a polydispersity of about 1.9, a weight average molecular weight of about 190,000, and furthermore, upon analysis, yielded a ratio of M.sub.z to M.sub.n of 3.04. Furthermore, this polymer was produced only in formulations bearing blowing agent in an amount of about 6 weight percent. In contrast, the polymer of the present invention has a combination of characteristics (polydispersity, weight average molecular weight, and M.sub.z :M.sub.n) which differs from the aforementioned commercially available polymer. Furthermore, the formulation of the present invention utilizes blowing agent in an amount of only from about 2 weight percent to about 4.4 weight percent.